Recently, vigorous research and development of a silicon photonics technique capable of drastically downsizing optical circuits are underway by applying a manufacturing technique of silicon integrated electronic circuits to the formation of optical waveguides and the like. Circuits having various functions are proposed as silicon optical circuits based on the silicon photonics technique, and development for their practical use is particularly conducted in the field of an optical transceiver.
FIG. 27A and FIG. 27B are diagrams each showing a typical configuration of an optical modulation circuit as a first example of a silicon optical circuit of a conventional technique. Each of optical circuits 9100-1 and 9100-2 in FIG. 27A and FIG. 27B is an optical modulation circuit chip of a digital coherent polarization multiplexed system which is applied to an optical transceiver mainly for long distance transmission. Both of the optical circuits include the same elements, which include an input waveguide 9101, optical splitters 9102 to 9108, optical phase modulation waveguides 9109 to 9112 configuring four Mach-Zehnder circuits, optical couplers 9113 to 9118, a polarization rotation circuit 9119, a polarization combining circuit 9120, and an output waveguide 9121.
The optical circuit 9100-1 of FIG. 27A is an example in which the input/output waveguides 9101, 9121 are arranged in the vicinity of two corners at a diagonal position of the chip such that the optical input/output are positioned at the both ends of the rectangular chip. The optical circuit 9100-2 of FIG. 27B is an example in which the input/output waveguides 9101, 9121 are arranged in the vicinity of the same corner such that the optical input/output are positioned at one end of the rectangular chip.
Although not explicitly illustrated in FIG. 27A and FIG. 27B, high frequency electrodes are formed on the upper part of each of the optical phase modulation waveguides 9109 to 9112 and are operated such that an electric signal is converted into an optical phase change (phase modulation signal) due to interaction between electricity and light. Light input from the input waveguide 9101 is sequentially branched by the optical splitters 9102 to 9108, and modulation is gained at the optical phase modulation waveguides 9109 to 9112. Further, modulated lights are merged by the optical couplers 9113 to 9118, the polarization rotation circuit 9119, and the polarization combining circuit 9120, and the resultant light is output from the optical output waveguide 9121 as a polarization-multiplexed optical modulation signal.
FIG. 28 is a diagram showing a configuration of an optical circuit in which an optical modulation circuit and an optical reception circuit are integrated as a second example of a silicon optical circuit of a conventional technique. An optical circuit chip 9200 is an optical circuit chip in which the optical modulation circuit of a digital coherent polarization multiplexed system and the optical reception circuit are integrated on a silicon substrate. The silicon optical circuit is excellent in that it has good integration properties and can suppress the size and cost of a circuit by integrating a plurality of functional circuits into one chip.
An optical modulator part located at the upper side of the integrated optical circuit 9200 of FIG. 28 has a configuration identical to that of FIG. 27B. The function and operation of respective circuit elements 9201 to 9221 are identical to those of the circuit elements 9101 to 9121 as illustrated in FIG. 27B. The optical reception circuit located at the lower side of the optical circuit chip 9200 is composed of a locally generated light input waveguide 9222, a signal light input waveguide 9223, an optical splitter 9224, a polarization separation circuit 9225, a polarization rotation circuit 9226, coherent optical mixers 9227, 9228 which are optical modulation circuits, and photo detectors (PD) 9229.
A polarization multiplex signal is input into the signal light input waveguide 9223 from a transmission path, and the polarization multiplex signal is separated by the polarization separation circuit 9225 into TE polarized light and TM polarized light components. Further, from a locally generated light source, continuous light of the TE polarized light is input from the input waveguide 9222 and branched into two by the optical splitter 9224. The TE polarized light component of a signal separated by the polarization separation circuit 9225 and a locally generated light of the TE polarized light, which is one of the branched light, are modulated by the coherent optical mixer 9227. Also, the TM polarized light component of a signal separated by the polarization separation circuit 9225 is converted into the TE polarized light by the polarization rotation circuit 9226, and is input to the coherent optical mixer 9228 together with a locally generated light of the TE polarized light, which is the other one of the branched light, for modulation. Thus modulated optical signals are converted into received electrical signals by the plurality of photo detectors 9229 and are output therefrom.